One type of conventional cooling system operates on the basis of a vapor compression refrigeration cycle. Such systems typically include a refrigerant compressor, an expansion valve, an evaporator, and a condenser, all connected in series in a closed loop. A cool air fan is used to blow air across the cooled surfaces of the evaporator. The cooled air is then directed into the interior of the structure to be cooled through to a cool air duct. A thermostat, sensing the temperature within the structure, actuates the cool air fan and the compressor during each cool demand cycle, which is triggered when the interior structure temperature is above a selected value.
With most cooling systems the cool air fan and the compressor turn on at the same time when the thermostat calls for cooling and run continuously during the entire cool demand period. Both the cool air fan and compressor turn off at the same time at the end of the cool demand period. In some cases, the switching off of the cool air fan can be delayed for a certain period to better utilize the coolness of the evaporator surfaces.
One problem with virtually all well-designed cooling systems is that they are designed to cool the structure to a chosen temperature, for example 78.degree., on the hottest days expected in that location. Cooling systems are therefore sized so the evaporator can absorb enough heat from the incoming air to deliver sufficiently cool air to the interior of the structure to cool the structure to the chosen temperature during the hot-test days. This leads to a cooling system which is oversized when operating at lower, and more common, temperatures. The excess capacity reduces the efficiency of the system for several reasons. The evaporator coils, during moderately hot days, remain cooler than they would be during the hottest days. Because the evaporator coils remain cooler, various portions of the system often ice up, especially in high humidity environments. This ice acts as an insulator, further reducing the efficiency of heat transfer between the air to be cooled and the evaporator. In some cases operating under these adverse conditions requires that an energy penalty of up to 50% be paid.
One method used to reduce icing has been to mount heater coils at appropriate places where icing may normally occur. However, this method is an obviously inefficient solution and does not address or solve the underlying problem.